The present invention relates to means of construction in heaving, water-saturated or non-cohesive grounds, including permafrost areas. More particularly, it relates to devices for securing in place underwater or underground structures and for improving the reliability of such structures, for example, pipelines in water-saturated or flooded areas, as well as pipeline sections subjected to variations in temperature and pressure, or those pipelines and/or sections movable under the effect of said factors, and it relates to their course of assembly.
Devices for securing pipelines in place, according to the present invention, can be used in industrial and civil construction and for building various pipelines, primarily, in the petroleum-and-gas industry for building high-pressure large-diameter gas- and oil pipelines, including those for the conveyance of cooled gas. The devices of the invention can be used most advantageously for securing in the design position gas- and oil pipelines of very large diameter (over 500 mm) laid out underground in unstable water-saturated grounds, for example, in peat bogs, in river plains, in areas of permafrost, the latter of which loses its supporting power upon defrosting and to areas subject to frost heaving, as well as in heaving grounds. It appears expedient to use the herein-disclosed devices for securing a pipeline in irrigated ground to increase the forces of cohesion of the pipeline with the ground, as well as in pipeline sections subjected to high temperature (for example, after a compressor station) and in non-cohesive grounds for ensuring the pipeline stability. The devices of the invention can be used to advantage in pipeline sections subject to shifting in the course of operation, as well as those shifted together with ballast means during assembly, for example, when assembling pipeline lengths by a floating technique, or by the gradual building up and ballasting with subsequent pulling through swamped areas. It is likewise expedient to use the disclosed devices for reducing the required amount of ballast in pipelines due to an increased cohesion of the pipeline with surrounding ground attained through the use of the devices of the invention.
Such devices can further be used for improving the reliability and stability of pipelines to avalanche destruction, especially those designed for conveying cooled gas.
With an increase of the pipeline diameter, as well as of the temperature of the product being conveyed and of the pressure inside the pipeline, the longitudinal forces along the pipeline tend to grow sharply while the specific forces of the pipeline cohesion with the surrounding ground decrease. In addition, the shifting of the bulk volume of present day pipeline construction to northern regions abounding in non-cohesive, finely dispersed, unstable and water-saturated grounds is further conducive to a sharp reduction in the holding power of the ground, particularly in those permafrost areas which thaw out in the warmer months of the year.
Under such conditions, trenches in which pipelines are laid frequently lack stable walls. The combination of all these factors has resulted in that large-diameter pipelines tend to shift in the ground during operation, both laterally and longitudinally. Prior art devices for securing pipelines, made as saddle-shaped ferroconcrete weights loosely hung on the pipelines, are unsuited for such operating conditions and tend to slide off the pipelines readily and damage the insulating coating, as well as fail to ensure the pipeline stability in the ground.
There are known in the art articulated weights comprising two ballast elements interconnected by means of a hinge. However, such weights tend to slide off the pipeline even more readily than the saddle-shaped ones since, upon lateral shifting of the pipeline, one half of the weight rests against the ground, transmitting part of the weight load thereto, while the other half generally tends to open up rotating around the hinge. In addition, the use of such weights requires wider trenches having a uniform plane bottom surface over a cosiderable width, which is practically impossible under conditions of swamped ground, and the use of rotary bucket excavators when digging trenches is prohibited.
Further studies resulted in the development of articulated weights, each comprising two identical elements encompassing the pipeline over more than a half of its circumferential perimeter and made fast on the pipeline by means of securing means including a wedge breechblock tightened by a screw joint. However, said weight failed to find application in construction due to the following disadvantages: the wedge block precludes the possibility of tightening the weight elements on the pipeline with a desired force; upon pressure variation in the pipeline, the tightening force tends to vary as well and to disappear gradually; the screw joint of the wedge breechblock often must be screwed underwater, which is rather inconvenient; it is practically impossible to check the tightening force; the removal of such weight during repairs is next to impossible for the screw joint is likely to corrode in the course of time and becomes very hard to unscrew; and the weight thereof also damages the insulation.
A weight has been developed recently, comprising two metal collars on which ferroconcrete elements are hung on both sides of the pipeline. However, these weight also suffers from a number of disadvantages, namely, inadequate ballasting power; the weight may tip over when resting against the ground and on shifting the pipeline to the side; considerably wider trenches are required, and it is impossible to use rotary bucket excavators for digging trenches; high steel consumption by the collars and the need to assemble the weight in situ; the need for two rigging men instead of one for assembly purposes; and the need for mechanical protection of the anticorrosion coating of the pipeline, as well as the possibility of damage to insulation due to the random manner in which the weight elements rest against the pipeline. While so doing, all of the afore-listed prior art devices for securing pipelines in place practically fail to attain any marked increase of the force of holding the pipeline in the ground or to ensure reliable operation of the pipelines.
In order to improve the stability of the attached weights, since the center of gravity of their mass tends to lower, as a result of which almost the entire mass of the weight gets sunk in water-saturated ground, this leading to the loss of up to 40% of the ballasting power of the weight, the use of a considerably greater number of weights is required. The stability of pipeline position in the ground in the absence of compensator means (most frequent scheme of operation) can be ensured with the aid of two factors, namely, ballasting the pipeline and increasing the force with which it is entrapped in the surrounding ground. However, none of the prior art weight structures provide a possibility of making use of the latter factor, that of increasing the pipeline entrapment force, which results in a sharp increase of the required ballast volume (by about 30%).
There is also known a method of coating pipes with concrete, however, this is a very costly method of securing pipelines, characterized by limited ballasting power due to a limited thickness of the concrete coating. This method further calls for changes in the construction process which is often impossible. At the same time, the weight of the pipes and the rigidity of the pipeline increase sharply, while the cohesion of pipeline with the surrounding ground increases but slightly. Therefore, the coating of pipes with concrete in swamped and flooded areas, especially in the North, is done as an exception to the general rule. This method is mainly used when laying a pipeline from a barge or when pulling a pipeline through a flooded trench from a stationary platform.
Prior art structures of ballasting devices fail to grip the pipeline tightly enough and, despite heavy material consumption, cannot be used with adequate efficiency as a means for precluding the avalanche destruction of pipelines.
Therefore, prior art devices for securing pipelines fail to provide reliable and efficient securing of pipelines laid out in swamps and flooded areas characterized by non-cohesive finely dispersed grounds.